Lithium secondary batteries are currently in widespread use as power sources for electronic devices such as portable telephones, video cameras, notebook computers, and so forth. In addition, with environmental protection and energy issues as a driving force, the development is also being pursued of inexpensive and very safe large lithium secondary batteries for electric automobiles and nighttime electrical power.
Layered rock salt-type LiCoO2 has primarily been employed for the positive electrode active material of lithium secondary batteries. LiCoO2 has excellent charge-discharge cycling characteristics, but is expensive due to the scarce reserves of its cobalt ingredient. This has led to investigations of layered rock salt-type LiNiO2 and spinel-type LiMn2O4 as replacement positive electrode active materials. However, LiNiO2 has a problem with the safety of its charged state, while LiMn2O4 has a problem with chemical stability in high temperature regions. Novel positive electrode materials that combine these elements been proposed for small batteries, but there has been demand for novel replacement materials for the positive electrode active material for large batteries, where the cost and safety requirements are more stringent.
LiFePO4 and LiMnPO4, which are olivine-type positive electrode active materials, have been under active development in recent years as materials that are well-rated with regard to cost, safety, and reliability. Many research reports on LiFePO4 have appeared in a short period of time because it has a better electroconductivity than that of LiMnPO4. LiMnPO4, on the other hand, has a higher energy density due to the higher redox potential of Mn and is looked upon as a positive electrode active material that has the potential to have better properties than LiFePO4; this notwithstanding, however, it has been reported to be a material that has a low electronic conductivity and is therefore problematic with regard to obtaining a satisfactory battery capacity. Efforts to improve the battery capacity by replacing some of the Mn with another element have also been proposed, as described in Patent documents 1 to 3 and Non-Patent documents 1 to 3. However, when the present inventors produced positive electrode active materials in which a portion of the Mn was replaced by a single selection from Co, Ni, Ti, and so forth as proposed in these patent references, and then fabricated batteries using these positive electrode active materials, the present inventors were unable to confirm an improvement in the capacity of these batteries. The present inventors were also unable to confirm a plateau at around 4 V in constant-current charge-discharge testing of these batteries.
There are also numerous publications relating to the addition of carbon to positive electrode active materials, and in particular it is already known that the addition of carbon has a number of effects in olivine-type lithium iron phosphate positive electrode active materials, such as improving the electroconductivity, inhibiting sintering between particles, inhibiting oxidation, and so forth (refer, for example, to Patent documents 4 to 6 and to Non-Patent documents 4 and 5). A problem here, however, is that the specific surface area of the positive electrode active material is increased by the addition of high specific surface area carbon particles and by the coating of the positive electrode active material by such carbon particles. This increase in the specific surface area causes a reduction in the dispersibility of the positive electrode active material in paint and thereby makes it difficult to uniformly coat the positive electrode active material at high densities on an electrode.
As shown in Patent document 7, a method directed to paint dispersibility has been proposed for the addition of carbon to positive electrode active materials. In this method, the particles of the positive electrode active material are coated with a thermosetting resin and the coated particles are then heat treated in an oxidizing atmosphere. However, a solvent is required to achieve uniform mixing and coating of the resin, and solvent handling is quite burdensome. In addition, an essential aspect of this method is heat treatment under an oxidizing atmosphere, which impairs the application of this method to olivine-type positive electrode active materials that contain a metal element that is easily oxidized from the divalent to trivalent state.
Patent document 1: Japanese Patent Laid-open Publication No. 2001-307731
Patent document 2: Japanese Patent Laid-open Publication No. 2003-257429
Patent document 3: Japanese Patent Laid-open Publication No. 2004-63270
Patent document 4: Japanese Patent Laid-open Publication No. 2001-15111
Patent document 5: Japanese Patent Laid-open Publication No. 2002-110163
Patent document 6: Japanese Patent Laid-open Publication No. 2003-34534
Patent document 7: Japanese Patent Laid-open Publication No. 2003-229127
Non-Patent document 1: by D. Arcon, A. Zorko, P. Cevc, R. Dominko, M. Bele, J. Jamnik, Z. Jaglicic, and I. Golosovsky, Journal of Physics and Chemistry of Solids, 65, 1773-1777 (2004)
Non-Patent document 2: A. Yamada, M. Hosoya, S. Chung, Y. Kudo, K. Hinokuma, K. Liu, and Y. Nishi, Journal of Power Sources, 119-121, 232-238 (2003)
Non-Patent document 3: Guohua Li, Hideto Azuma, and Masayuki Tohda, Electrochemical and Solid-State Letters, 5(6), A1135-A1137 (2002)
Non-Patent document 4: H. Huang, S. C. Yin, and L. F. Nazar, Electrochemical and Solid-State Letters, 4(10), A170-A172 (2001)
Non-Patent document 5: Z. Chen and J. R. Dahn, Journal of the Electrochemical Society, 149 (9), A1184-A1189 (2002)